Refining maleic acid



Patented July 17, 1934 PATENT OFFICE REFINING MALEIC ACID Michael N.Dvornikoff, St. Louis, Mo., assignor, by mesne assignments, to MonsantoChemical Company, a corporation of Delaware No Drawing. ApplicationFebruary 9, 1931, Serial No. 514,711

2 Claims.

This invention relates to the treatment of maleic acid whereby a crudeproduct may be purified economically and without excessive byproductformation.

This application is .a continuation in part of my co-pendingapplication, S. N. #439,263, filed March 26, 1930.

Maleic acid undergoes intermolecular rearrangement, forming fumaricacid, when it is exposed to an elevated temperature. The rate of thisrearrangement increases as the temperature is increased.

A further obstacle to the successful distillation of maleic acid isintroduced by reason of the fact that when maleic acid is maintained attemperatures even as low as 75 C., it decomposes into maleic anhydrideand water. At this temperature, the vapor pressures of both the waterand anhydride are very appreciable. In an effort to remove the maleicacid rapidly from the zone of heating and thereby reduce fumaric acidformation, it has been proposed heretofore to distill maleic acid underreduced pressure with the result that the maleic acid, largely in theform of maleic anhydride and water, passes directly from solid to vaporphase. The fact that the remaining material is in a dry conditionimpairs the heat transfer, exposes the acid to prolonged hightemperature, pro motes local overheating and renders the process of nopractical value.

To overcome these disadvantages, it has been proposed that maleic acidbe distilled in the presence of an inert nonaqueous liquid mediumwhereby the water distills off as formed with the inert liquid, leavingthe anhydride dissolved or suspended, after which it is separatedtherefrom by fractionation or distillation. This process suffers theinconveniences of relatively large quantities of a foreign constituentas well as the need of extensive equipment for the refining of acomparatively small output of product.

In my copending application, I have disclosed a method of treating crudemaleic acid whereby substantial yields of maleic anhydride may beobtained. Briefly stated, the process consists in distilling maleic acidunder prescribed conditions and fractionally condensing the vapors. Theless volatile maleic anhydride condenses whereas the Water vapor andremaining maleic anhydride are conducted to a separate condensing systemin which completev condensation of the remaining vapors is effected Whenmaleic acid is desired in lieu of maleic anhydride, fractionalcondensation is not practiced and the entire charge is condensed in asingle receptacle resulting in the formation of the acid directly.

I have found that by controlling the conditions of distillation wherebya liquid phase is maintained in the still preferably by adding the crudeacid progressively to the charge and by limiting the quantity of acidundergoing treatment at any time as well as restricting as much aspossible the time of exposure of the acid to elevated temperatureconditions, one is enabled to refine maleic acid effectively and obtainapure product which is substantially free from the usual impurities,while at the same time avoiding to a very large extent, if not en- 0tirely, fumaric acid formation.

The process may be operated either as a continuous, semi-continuous, orstrictly batch process, one characteristic feature of all being," thepresence of a liquid phase in the still during the distillation.

Since under the conditions of the distillation, the temperature issufiiciently high to promote fumaric acid formation, the quantity ofmaleic acid undergoing distillation at any time is restricted.Nevertheless, fumaric acid may be formed and is disposed of by drainingthe still periodically or removing such residue in any other manner. Astirring mechanism should be employed for the purpose of preventinglocal overheating and otherwise facilitating the heat transfer from thevessel to the liquid; also to promote dissolving or' melting of thesolid in the liquid.

'The maintenance of the liquid phase is dependent upon a number offactors, notably the temperature of the vessel, the capacity of theequipment to transfer heat from the source thereof into the charge, thepressure within the vessel, the quantity and nature of the impurities inthe crude maleic acid and other factors.

With practice, the liquid phase can be maintained without difiiculty. Ingeneral, other conditions being favorable, the temperature should bemaintained as low as possible While maintaining the presence of a liquidphase in an amount sufficient to gain the advantages of a heat transferfrom the wall of the vessel to the solid maleic acid through a liquidphase rather than directly from the vessel to the solid acid.

With most types of crude acid, both extremes in temperature as well aspressures are to be avoided.- Thus even when heating the vesselvigorously, it is possible to reduce the pressure suificiently to effecttotal disappearance of the 110 liquid phase, notwithstanding the factthat there is at all times an excess of solid phase. After the moltenphase has once disappeared, the heat transfer to the solid material isreduced to such an extent that local overheating and the formation offumaric acid exceeds the bounds of commercial practicability. Facilitiesand conditions for improving the rate of heat transfer from the vesselto the charge including adequate agitating means, large exposed heatingsurface as well as others well known to those skilled in the art, are tobe favored. Various pressure and temperature conditions will be foundoperative, depending in each instance to some extent upon the purity ofthe product being treated, the nature of the equipment, etc.

Examples of the application of the principles of my invention arehereinafter set forth. Maleic acid is heated to about 110 C. in areceptacle provided with an agitating mechanism adapted to be operatedunder reduced pressure conditions. As this temperature is attained, atleast a portion of the acid will liquefy and the charge as a wholeattains a mush consistency. The product is distilled under reducedpressure conditions, 50-60 mm. having been found suited to thisparticular temperature. Crude acid is added progressively during thedistillation to replenish that which is distilled. If maleic anhydrideis desired as one of the products, a fractional condensation may beundertaken. If, on the other hand, the acid alone is required, a singlecomplete condensation will be found satisfactory.

Another procedure which exemplifies the principles of the inventionconsists in charging an enamel steam jacketed kettle, which is providedwith an agitator, with a small amount of the crude material containing-95 per cent acid product. The charge is heated by means of steam orother heating medium within the jacket and at the same time it is slowlyagitated. After a liquid phase appears, the temperature in the jacket isadjusted to correspond to that of steam at 25-30 pounds pressure(130-135 C.) and the pressure within the kettle is reduced to 30-50 mm.Crude maleic acid is added continuously or periodically to the still atsuch a rate as to replenish the acid that is distilled. The evolved acidvapors probably consisting mainly of maleic anhydride and water arecondensed in any desired manner. One convenient method consists inbringing the vapors in contact with a saturated aqueous solution ofmaleic acid from which the refined acid is precipitated in crystallineform and is separated and dried by the ordinary methods.

What I claim is:

1. The method of refining crude maleic acid that comprises distillingthe same in the absence of an organic solvent at a temperature notsubstantially in excess of 135 C. and under subatmospheric pressureconditions while maintaining a -liquid phase of the acid in the still bysubatmospheric pressure and temperature control.

2. The method as defined in claim 1 and further characterized in thatthe temperature of distillation is greater than 100 C. but notsubstantially above 135 C. and the pressure not substantially more than60 mm.

MICHAEL N. DVORNIKOFF.

